No. Nightmare difficulty is there for only the most dedicated and extreme players who want a serious challenge so it should be significantly tougher than hard. When it comes to harder than hard mode difficulties I believe it should lean towards being too difficult as opposed to too easy.

Toal has a line in his description saying that it's for good players only.

He is harder than the others, and if you add the step up of dificulty too, you end up getting hard time even on normal mobs from the get-go.Admitedly, I only completed the game with yunica on normal prior to trying this, and ended up abusing the invulnerability from the thunder spell, but even the wind spell seems way less useful (no area of effect, forced move).

I guess Toal just needs us to get used to his very close combat style.

Toal is a gigantic step up from Yunica/Hugo in difficulty- likely for the first boss alone, who is way harder than the bat ♥♥♥♥♥♥. But, yeah, levelling up makes a gigantic difference in this game- just having +1 defence can take those 7 hits down to 3 or 2, which really adds up. Getting weapon upgrades is pretty much compulsory, too.

Make sure you're at least at level for every boss you run across (check the bestiary). One of Nightmare's main gimmicks is to considerably inflate the effect of level discrepancy on the damage you do and take. If you're still having problems, even grinding out one level over them will tremendously reduce the damage you take.

Also, Galleon is actually quick enough to mess you up and doesn't follow a really easy pattern like the other first midbosses. Hope you learned that dash move really fast, because you're immune to damage while using it and you have enough of a window to squeeze in just a couple hits between his attacks.

Also Hugo is kind of an easymode character for most fights and not even Nightmare boss rush is all that difficult for him. Toal actually has some hard fights that he can't cheese with an overpowered shield and a ranged attack, though some of the later ones are kind of trivialized by his fire attack (Pictimos, Zava, and Dalles are made hilariously easy, even on Nightmare).